The quality of the photochromic lens is determined by the speed of the lens to change its color and its life span. For a photochromic lens, it should change its color quickly enough otherwise the eyes will feel uncomfortable. Certainly a lens which changes its color too fast is not always the best for it may turn to be colorless at a high temperature thus will fail to fulfill its duty to block glare. Another important property of the photochromic lens is its life span. A quality photochromic lens should have a long life span which should be no less than the life span of the lens material itself. While for lens with low quality, it can only last for several months or even several days before the lens stops changing its color. Besides, the base color of a quality lens should relatively light to ensure the transparency of the lens indoors is good enough. If a photochromic lens is cracked or deformed after a while, it means that the lens is of low quality.